Twenty-one people have been killed and 41 injured after a bomb exploded in an airliner at Colombo airport in Sri Lanka.

There were 128 passengers on board - among them 25 Britons, five of whom were injured. It is believed the bomb was planted by Tamil rebels seeking a separate homeland on the island state.

The Air Lanka Tristar had come from London's Gatwick airport, stopping at Zurich and Dubai on the way to the Sri Lankan capital, Colombo.

Flight UL512 named the 'City of Colombo' was carrying mainly French, British and Japanese tourists and was about to fly on to the Maldives Islands in the Indian Ocean when the explosion ripped the plane in two.

Windows in the terminal building were shattered.

"All of a sudden there was a massive, massive flash bang with flames," said one of the British survivors, Simon Ellis.

"The ceiling came down and our chair was blown backwards. When I managed to climb over the chairs, I looked out and there it was - there was nothing. The plane had been blown in half just right behind our chairs."

The attack was probably designed to damage peace talks brokered by India between the Tamil rebels and the Sri Lankan government.

The security forces in Sri Lanka said they had already had warnings from Tamil guerrillas that they were planning a major attack in the capital.

Sri Lankan officials believe the bomb may have been concealed in crates of meat and vegetables being freighted to the Republic of Maldives, but no official announcement has been made.

A search of the aircraft the next day uncovered a parcel containing uniforms with the insignia of the Black Tigers, the commando wing of the Liberation Tigers of Tamil Eelam (LTTE), the fiercest of the Tamil guerrilla groups.

Among the dead were two Japanese, a French citizen, three Ceylonese and two people from the Maldive Islands, where the plane was going. Three of the others were believed to be Britons.

The tail was blown off, as the bomb was hidden in a tool box which was located in the rear section of the cabin. This flight had been delayed, and if it had departed on time, everybody on board would have been killed, and the cause would have been difficult to determine as it would have flown overwater.

Passengers were boarding the AirLanka L-1011 at 9.10 a.m., 20 minutes before the scheduled departure, when the explosion occurred, state radio said.

A western pilot who spoke on condition of anonymity said he saw bodies lying on the tarmac while cargo, such as chickens and vegetables, was strewn over the area. Passengers were running and screaming, he said.

One injured passenger, a French sailor, told reporters he had not yet taken his seat when the explosion rocked the aircraft.

"I was lucky to escape alive," said the man who would identify himself only as Redato.

The airport was closed for four hours after the blast.

The Lockheed Tristar arrived from London via Zurich, Switzerland, and Dubai in the United Arab Emirates. Passengers had left the plane so it could be cleaned and refuelled.

Reports said 128 passengers were scheduled to reboard for a flight to Male in the Maldive Islands, 388 nautical miles southwest of India's southern tip.

Among the passengers were people from Britain, Germany, Austria, France, Italy, Japan, India, Sri Lanka and the Maldives.

A Japanese foreign ministry official in Tokyo said a Japanese newlywed couple was feared killed. Two other Japanese were injured and four others were reported safe, the official said.